1. Field of the Invention
The present invention relates to a solenoid valve in which a fluid flows at substantially the same flow rate in both directions of a flow path.
2. Description of the Related Art
Heretofore, an oxygen generating apparatus for medical use has been known in which two adsorption columns are provided which adsorb nitrogen from compressed air in order to generate concentrated oxygen. With this type of oxygen generating apparatus, using the two adsorption columns alternately, so that concentrated oxygen can be generated from the compressed air continually over an extended period of time, during the time that nitrogen is being adsorbed by one of the adsorption columns, nitrogen is removed from the other of the adsorption columns to enable regeneration of the other adsorption column. In this manner, when the two adsorption columns are switched and used alternately, a pressure equalizing valve is provided so that the pressures of the flow paths that are connected to the adsorption columns can be equalized rapidly. Moreover, during the time that the other of the adsorption columns is regenerating while nitrogen is being adsorbed by the one adsorption column, a portion of the concentrated oxygen that is generated in the one adsorption column is supplied to the other of the adsorption columns through an orifice that is formed in a passage that bypasses the pressure equalizing valve.
In addition, as the structure of the pressure equalizing valve, a symmetrical configuration is sometimes adopted, in which two solenoid valves of the same structure are disposed in a manifold block. Thus, in the case that the adsorption column that is used for generating concentrated oxygen is switched from one to the other, or is switched from the other to the one, while taking into consideration the fact that the direction of flow of the fluid is reversed, a constant flow rate is ensured in either of these cases.
However, as stated above, in the manifold type pressure equalizing valve, which is equipped with two solenoid valves, an increase in the size of the apparatus cannot be avoided. Further, the flow passages formed in the manifold block are complex, and as a result, a problem is noted in that the oxygen generating apparatus as a whole is unavoidably expensive.
Thus, for example, a pressure equalizing valve for an oxygen generating apparatus has been proposed, as disclosed in Japanese Laid-Open Patent Publication No. 2001-090860. In the pressure equalizing valve, for addressing a problem that occurs in the case that one solenoid valve is provided, or more specifically, in which the flow rate differs depending on the direction in which the fluid flows, in the event that the valve is opened in opposition to a high pressure, a current when the valve body is attracted is set based on the measurement of a current and an energizing time necessary for such a fluid pressure, whereby the attraction force of the valve body is increased, and after attraction of the valve body, a predetermined current is made to flow, which is as small as possible needed to maintain the attracted state. Consequently, even if a small size solenoid valve is used, a large current is made to flow and a large attraction force can be obtained only during attraction of the valve body, and in the case that a fluid flows in both directions with respect to the single solenoid valve, opening and closing of the valve can be controlled so that, in either of such directions, the flow rate is kept constant without being affected by the fluid pressure.